Why Concrete Cracks: 7 Types, Causes, Prevention & Repair

What They Are

Plastic shrinkage cracks appear within 30 minutes to 6 hours after placement, while concrete is still in plastic (fresh) state. They occur when the rate of surface evaporation exceeds the rate of bleed water rising to the surface.

Appearance

• Usually 50-100 mm deep (sometimes through full slab thickness)
• Typically 300-600 mm long
• Random or diagonal pattern
• Often parallel, spaced 50-100 cm apart

Causes

• High evaporation rate (>1.0 kg/m²/hr)
• Low humidity combined with high temperature
• Strong winds at concrete surface
• Direct sunlight on fresh concrete
• Delayed finishing operations

Prevention

• Use evaporation retardants (monofilms)
• Erect windbreaks and sunshades
• Start curing immediately after finishing
• Avoid concreting during hottest hours
• Fog spraying during finishing

What They Are

Drying shrinkage cracks develop weeks to months after construction as concrete loses moisture to the environment. They result from the volumetric contraction of hardened concrete.

Appearance

• Usually vertical in walls, diagonal in slabs
• Typically extend through full thickness
• Often at regular intervals following reinforcement pattern
• Width varies with restraint conditions

Causes

• High water-cement ratio
• Inadequate curing
• High cement content
• Large aggregate size (insufficient paste)
• Structural restraint preventing movement

Prevention

• Use lowest practical w/c ratio
• Extended moist curing (minimum 7-14 days)
• Provide adequate contraction joints
• Use shrinkage-compensating cement where appropriate
• Add fibers to control crack widths

What They Are

Thermal cracks result from temperature differentials within concrete mass or between concrete and ambient conditions. Common in mass concrete and hot/cold weather conditions.

Types of Thermal Cracking

1. Early-age thermal cracks: Due to heat of hydration in mass concrete
2. Surface cracks: Rapid cooling of surface while core remains hot
3. External restraint cracks: Thermal contraction restrained by foundation or adjoining elements

Appearance

• Usually deep and through-going
• Regular spacing pattern
• More common in thick sections (>500mm)

Prevention

• Use low-heat cement (PPC, PSC)
• Limit pour heights and layer thickness
• Cool aggregates and mixing water
• Insulate formwork to reduce temperature gradient
• Provide cooling pipes in mass concrete

What They Are

Settlement cracks occur due to differential settlement of the supporting soil or subgrade. They can also form when concrete settles around embedded objects like reinforcement.

Appearance

• Usually diagonal or stepped pattern
• Often at building corners or changes in foundation depth
• Wider at top, tapering down
• May extend through walls and foundations

Causes

• Uneven soil bearing capacity
• Inadequate soil compaction
• Changes in moisture content of expansive soils
• Overloading of foundations
• Poor drainage around foundations

Prevention

• Proper soil investigation and foundation design
• Adequate compaction of backfill
• Plinth beam at foundation level
• Proper drainage around structure
• Expansion joints in expansive soil areas

What They Are

Structural cracks indicate overstressing of concrete elements due to loads, settlement, or design deficiencies. These require immediate engineering assessment.

Types and Appearance

Action Required

• Document crack width, length, and pattern
• Monitor for progressive movement
• Engage structural engineer for assessment
• Implement temporary support if necessary

What They Are

Cracks resulting from internal chemical reactions that cause expansion within concrete:

1. Alkali-Aggregate Reaction (AAR)

• Alkali-Silica Reaction (ASR): Most common, reactive silica in aggregates
• Alkali-Carbonate Reaction: Certain dolomitic limestones
• Appears years after construction
• Map-like cracking pattern with gel exudation

2. Sulfate Attack

• External sulfates from soil or groundwater
• Internal sulfates from contaminated aggregates
• Causes expansion and deterioration
• White deposits on surface

3. Carbonation

• CO₂ penetration reduces concrete pH
• Depassivates reinforcement leading to corrosion
• Corrosion products cause expansion and spalling

Prevention

• Use non-reactive aggregates (petrographic testing)
• Limit alkali content in cement (<0.6% Na₂O eq)
• Use sulfate-resisting cement in aggressive environments
• Ensure adequate cover and low permeability

What They Are

Crazing is a network of fine cracks on the surface, creating a pattern resembling spider webs or alligator skin. Usually a surface phenomenon with no structural significance.

Appearance

• Fine, shallow cracks (less than 1mm deep)
• Hexagonal or irregular pattern
• More visible when surface is wet
• Most common on troweled surfaces

Causes

• Over-finishing or over-troweling
• Rich mix (high cement content)
• Rapid surface drying
• Hot weather during finishing
• Excessive bleeding followed by rapid drying

Prevention

• Avoid over-troweling
• Use moderate cement content
• Cure properly
• Apply evaporation retardant in hot weather